1 Technical Field
The present invention relates to AC (Alternating Current) excitation synchronous rotating electric machines which include, at least, a multi-phase coil and an armature core, but no field winding.
2 Description of Related Art
To achieve a small size, high performance, long service life and high reliability, rotating electric machines generally employ a brushless structure with a permanent magnet field. On the other hand, for usage in a wide rotational speed range, it is necessary to vary the field strength. However, with the aforementioned permanent magnet field, it is difficult to vary the field strength; thus losses occur and there are limits on the characteristics of the rotating electric machines. Therefore, one may consider employing a winding field instead of the permanent magnet field. However, the space required for receiving a winding is generally larger than that required for receiving a permanent magnet. Moreover, a winding is generally required to be wound on a core. Consequently, a considerably large volume is required for employing the winding field. As a result, it is difficult to achieve the original object, i.e., to achieve a small size and high performance.
Japanese Patent Application Publication No. JP2000041367A discloses a hybrid excitation synchronous rotating electric machine developed for reducing the overall size, preventing occurrence of magnetic saturation and making high-speed rotation possible. Specifically, in the hybrid excitation synchronous rotating electric machine, a rotor is disposed radially outside a stator so that a back yoke portion of the rotor faces armature cores of the stator with a radial gap formed therebetween. Moreover, the rotor further has a plurality of N-pole permanent magnets, a plurality of S-pole permanent magnets, a first group of core salient poles and a second group of core salient poles, all of which are provided on a radially inner periphery of the back yoke portion of the rotor. The N-pole permanent magnets are arranged alternately with the core salient poles of the first group in a circumferential direction of the rotor. The S-pole permanent magnets are arranged alternately with the core salient poles of the second group in the circumferential direction of the rotor. Furthermore, each of the N-pole permanent magnets is out of axial alignment with any of the S-pole permanent magnets.
However, the hybrid excitation synchronous rotating electric machine disclosed in the above patent document necessitates an excitation winding in addition to an armature coil. Specifically, in the hybrid excitation synchronous rotating electric machine, the excitation winding is embedded in a back yoke portion of the stator. Accordingly, it is necessary for the back yoke portion of the stator to have a considerably large volume so as to allow the excitation winding to be embedded therein. Consequently, it is difficult to achieve a small size of the hybrid excitation synchronous rotating electric machine. In addition, additional manufacturing time is needed for forming the excitation winding and embedding the excitation winding in the back yoke portion of the stator.